The present invention relates to a rotation detector, and more specifically, to a rotation detector including a rotor having magnets.
FIG. 1 illustrates how an absolute position detection type detector detects rotational position. The rotation detector includes a rotor 30, which is fixed on a rotating shaft and integrally rotates with the shaft. An N pole zone 32 and an S pole zone 33 are alternately formed at sixty degree intervals on the rotor 30 in a circumferential direction. In positions facing the rotor 30, first to third magnetic resistance elements 31 are arranged around the axis O of the rotor 30 at forty-degree intervals. Each of the first to third resistance elements 31 detects the N pole zone 32 and the S pole zone 33, which alternately pass by the resistance elements 31 at sixty-degree intervals during the rotation of the rotor 30.
When the N pole zone 32 is detected, the first to third resistance elements 31 respectively output signals SG1, SG2, SG3 having an H level. When the S pole zone 33 is detected, the resistance elements 31 respectively output the signals SG1, SG2, SG3 having an L level. When the zone detected by each resistance element 31 moves from the N pole zone 32 to the S pole zone 33, each of the signals SG1, SG2, SG3 changes from the H level to the L level. Contrarily, when the zone detected by each resistance element 31 moves from the S pole zone 33 to the N pole zone 32, each of the signals SG1, SG2, SG3 changes from the L level to the H level. As shown in FIG. 1, the signals SG1, SG2, SG3 of the resistance elements 31 change gradually between the L and H levels. The reason for this is because a direction of magnetic flux changes gradually when the detected zone moves from the N pole zone 32 to the S pole zone 33. Three comparators (not shown) respectively receive the signals SG1-SG3 and adjust the waveforms of the signals SG1-SG3, thus generating detection signals S1-S3, which change sharply between the L and H levels.
More specifically, each of the comparators compares an output signal with a reference value, which is a middle level between the H level and the L level, and generates an H level detection signal S1-S3 when the output signal is greater than the reference value or generates an L level detection signal S1-S3 when the output signal is lower than the reference value. The reference value is the level of the signals SG1-SG3 output when the border between the N pole zone 32 and the S pole zone 33 passes by each of the first to third resistance elements 31. When any one of the detection signals S1-S3 changes from the L level to the H level or from the H level to the L level, the rotational position of the rotor 30 (or rotation shaft) is determined based on the state of the other detection signals. In the case of FIG. 1, the rotational position (absolute position) is detected in the range of zero to 120 degrees at intervals of twenty degrees.
However, it is difficult to precisely form the N pole zone 32 and the S pole zone 33 alternately at sixty-degree intervals on the rotor 30 in the circumferential direction. Accordingly, the rotational position is not detected at twenty-degree intervals with precision at the point when the detection signals S1-S3 change from the L level to the H level or from the H level to the L level.
Furthermore, in the above rotation detector, the levels of the output signals SG1-SG3 may be varied by objects located near the resistance element 31 that can affect magnetic flux. A shielding member may be provided to shield the rotation detector. However, this increases the number of parts, cost and assembly work.
The objective of the present invention is to provide a rotation detector that detects rotational position with high precision.